This invention relates to a noise reduction circuit with monitoring functionality, particularly but not exclusively for a headphone.
Headphones with passive noise reduction are usually equipped with ear cushions that completely enclose the ears (i.e. circumaural) and the cushions provide passive reduction or isolation from ambient noise. The extent of reduction is largely dependent on the nature of the ambient noise and the acoustics characteristics of ear cushions of the headphone. Due to the characteristics of the ear cushions, most passive noise reduction headphones attenuate the higher frequency components (approximately from 200 Hz and above) of the ambient noise, and the lower frequency components would still be heard by a user of the headphone. As a result, such passive headphones may not provide sufficient or effective noise reduction in certain noisy environments.
To address the above problem, active noise reduction circuits have been provided in headphones and such circuits are configured to eliminate or attenuate lower frequency components of the ambient noise to result in more effective noise attenuation. Ideally, ambient noise waveform is detected and an identical anti-noise waveform, which is equal in magnitude, but of opposite polarity is produced. Interaction of the noise waveform with the anti-noise waveform results in cancellation of the noise waveform.
It is an object of the present invention to provide a noise reduction circuit with monitoring function which provides a useful alternative to similar known circuits.